GRAIL set for launch on innovative moon mission, weather permitting

KENNEDY SPACE CENTER, FL--A United Launch Alliance Delta 2 rocket is on track for launch Thursday, weather permitting, to kick off a $496 million mission to precisely map the moon's gravitational field using two small satellites that will orbit in tandem to probe the hidden interior of Earth's nearest neighbor.

A United Launch Alliance Delta 2 rocket undergoes final preparations for launch Thursday on a mission to propel NASA's GRAIL gravity mapping satellites to the moon. (Justin Ray/Spaceflight Now)

The Gravity Recovery and Interior Laboratory, or GRAIL, "is a journey to the center of the moon," said Ed Weiler, NASA's associate administrator for space science. "It will probe the interior of the moon and map its gravity field 100 to 1,000 times better than ever before. We will learn more about the interior of the moon with GRAIL than all previous lunar missions combined."

Liftoff from launch complex 17B at the Cape Canaveral Air Force Station is targeted for 8:37:06 a.m. EDT (GMT-4). Another "instantaneous" one-second launch window is available at 9:16:12 a.m. Forecasters are predicting a 60 percent chance of a delay Thursday or Friday due to electrically active anvil clouds to the west and thunderstorms to the east. Conditions are expected to improve this weekend.

This will be the 110th and final planned Delta 2 fight from complex 17. While ULA has enough hardware on hand to build a final five medium-lift Delta 2s, any future flights of the workhorse rocket almost certainly will be launched from the West Coast.

As such, the GRAIL launch marks the end of an era at Cape Canaveral after more than 22 years of memorable Delta 2 flights, including 49 Global Positioning System navigation satellites and all of NASA's recent Mars missions, including the enormously successful Spirit and Opportunity rovers.

Equipped with nine strap-on solid-fuel boosters for extra power, GRAIL's Delta 2 will boost the twin spacecraft on looping 2.6-million-mile voyages to the moon, a round-about low-energy trajectory that allows the use of a lower-cost, medium-lift rocket.

The flight plan calls for the GRAIL-A spacecraft to brake into a highly elliptical 11.5-hour orbit around the moon's poles on New Year's Eve with GRAIL-B following the next day. Over the next two months, flight controllers will carefully maneuver the two satellites into the same circular orbit at an altitude of about 34 miles.

If all goes well, science operations will begin around March 8. Flying in formation at distances ranging from about 46 to 140 miles, the two spacecraft will send radio pulses and timing signals back and forth to precisely measure the distance between them and their relative velocity.

Sailing over hidden mass concentrations, mountains, basins and other geologic features, the satellites will ever so slightly speed up and slow down, one after the other. The ranging system is accurate enough to detect differences in distance that amount to the width of a red blood cell.

By carefully analyzing those changes, scientists can determine the distribution of mass within the moon to gain insights into its hidden interior structure and the nature of its core. In so doing, researchers may be able to confirm or rule out theories about the moon's formation and evolution and, by extension, improve understanding of the early histories of other terrestrial planets.